On a trip that we make frequently, my husband and I switch off on who drives up and who drives back. Even if I drive in the heat, I have so much gas left over when I drive at the speed limits. When my husband drives and even if it's at night and the temperature is cooler, he uses almost the whole tank. I've normally only used half. There is a difference when you slow down, but I also read somewhere that you have to go at least 35 or 40 miles to be gas efficient.
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